The preparation of intricately formed three-dimensional objects using stereolithography has been known for a long time. The desired object is usually built in a container which contains a radiation-curable liquid formulation (hereinafter also termed "stereolithographic resin") by a repeated alternating succession of two process steps (a) and (b), wherein in step (a) a layer of the radiation-curable liquid formulation, one limitation of which is the surface of the formulation, is cured by suitable radiation using usually a, preferably computer-controlled, laser source within an area which corresponds to the desired cross-section of the object to be built at the level of this layer, and wherein in step (b) a new layer of the radiation-curable liquid formulation is formed on said cured layer.
In some cases it is advantageous to use a radiation-curable liquid formulation which contains fillers.
As is disclosed, inter alia, in U.S. Pat. No. 4,942,066, e.g. specific fillers can be used to advantageously control the depth of penetration of the laser beam into the stereolithographic resin, which laser beam is usually used for curing the radiation-curable liquid formulation in the stereolithographic process described above.
It is also known that fillers, for example Al.sub.2 O.sub.3 or graphite, reduce the loss in volume during the curing of a stereolithographic formulation and therefore result in stereolithographically produced objects which are formed particularly accurately (T. Nakai and Y. Marutani; Reza Kenkyu, January 1988, pages 14-22).
Furthermore, JP-A-247515/1985 describes the addition of finely particulate materials, such as pigments, powdered ceramic and powdered metal, to stereolithographic resins in order to produce decorative effects, lower abrasion or electric conductivity in stereolithographically produced objects.
EP-A-0 250 121 discloses the addition of filler particles to a stereolithographic resin, which are transparent to the radiation used for the curing, in order to reduce the amount of the resin material which needs to be cured, resulting in a reduction of the radiation dosage required for the curing, i.e. in an increase of the radiation sensitivity of the stereolithographic resin.
However, working with filler-containing stereolithographic resins such as the ones described above, often leads to problems because, depending on the density of the filler and of the liquid residual stereolithographic resin, after some time the fillers either float to the surface or they sediment in the stereolithographic resin bath. Because of this, the filler content of stereolithographic resin in the bath changes very quickly, in particular where there are substantial differences in density between filler and residual resin and especially in the area close to the surface of the stereolithographic resin bath which is important for stereolithography. If filler and residual resin are of different density it is therefore in many cases not possible to obtain objects having material properties which are sufficiently uniform because the properties of stereolithographic resin on the one hand, e.g. its radiation sensitivity, and, on the other hand, those properties of the cured material which depend on the fillers, e.g. on its conductivity or mechanical properties, can change in uncontrolled manner.
The invention provides a generally usable and technically safe solution to said problem, which can be used for stereolithographic resins of any composition which can also contain any fillers, including fillers having a relatively large particle size and which therefore sediment quickly, or which have a density which is substantially larger than the customary density of the residual stereolithographic resin, which is usually in the range of only 1 to 1.3 g/cm.sup.3. Accordingly, this invention enables the skilled person to choose fillers solely according to the purpose envisaged therefor without having to fear technical problems or uncontrolled deviations in the quality of the stereolithographically produced objects.